Photopolymerizable composition for cladding optical fibers

ABSTRACT

A photopolymerizable composition for cladding optical fibers comprises an unsubstituted or fluorosubstituted diacrylate monomer; a fluorinated monofunctional acrylate monomer in an amount of from about 2 to about 12 parts by weight per part by weight of the diacrylate monomer; a photoinitiator; and a viscosity modifying agent to increase the viscosity of the composition to about 1000 to about 15000 cP. Upon photocuring, the composition has a refractive index not greater than about 1.43, and preferably not greater than about 1.40.

BACKGROUND OF THE INVENTION

This invention relates to a photopolymerizable composition for claddingoptical fibers.

As discussed in, for example, Blyler and Aloisio, Polymer coatings foroptical fibers, Chemtech, November 1987, pages 680-684, optical fibersconsist of a central core, usually a highly transparent glass (silica,often containing various doping materials) surrounded by a cladding witha refractive index lower than that of the glass; this cladding serves toconfine light within the central core in order to reduce radiationlosses from the surface of the core, and hence reduce attenuation ofradiation travelling along the core.

In most optical fibers, the cladding is formed from a second glass.Because minor flaws in such a glass cladding greatly reduce the tensilestrength of the optical fiber, it is customary to provide the fiber witha secondary, protective cladding, which is usually formed from apolymeric material. For example, U.S. Pat. No. 4,558,082, issued Dec.10, 1985, and U.S. Pat. No. 4,663,185, issued May 5, 1987, describeacrylated silicone polymers useful as, inter alia, optical fibercladdings. These silicone polymers are prepared by reacting limoneneoxide-functional silicones with acrylic acid or a substituted acrylicacid in the presence of a catalyst, which can be a tetraalkylurea or atetraalkylguanidine.

Acrylic resins have also been used as protective claddings for opticalfibers. U.S. Pat. No. 4,427,823 describes an uncured, filled coatingcomposition comprising 100 parts by weight of (a) a polyfunctionalacrylic-type carboxylic acid ester monomer or its prepolymer, thismonomer or prepolymer being composed of 0 to 75 percent by weight tri-or higher acrylates, and 25 to 100 percent diacrylate; (b) 0.001 to 20parts by weight of a polymerization initiator; and (c) 5 to 250 parts byweight of an inorganic solid filler having a refractive index of 1.40 to1.60 and an average first-order particle diameter of at least 1 mμ butless than 1 μ.

U.S. Pat. No. 4,479,984, issued Oct. 30, 1984, describes multifilamentbundles (which can be optical fiber bundles) impregnated with anultraviolet curable resin to form a composite material suitable for useas a strength member. Among the resins which can be used in such bundlesare various acrylate resins.

U.S. Pat. No. 4,690,503, issued Sept. 1, 1987, describes a glass opticalfiber having a primary coating constructed of two layers of ultravioletcured acrylate resin. The first, inner layer has a modulus of elasticityat 25° C. less than or equal to 5 N/mm²., while the second, outer layerhas a modulus of elasticity at 25° C. of from 25 to 1500 N/mm²., theratio of the thickness of the first layer to the thickness of the secondlayer being between 0.5 and 2.

U.S. Pat. No. 4,572,610, issued Feb. 25, 1986, describes the cladding ofoptical fibers by contact with a coating composition comprising (a) aradiation-curable diethylenically unsaturated polyurethane resinconstituted by an essentially saturated, halogenated,dihydroxy-terminated linear liquid polybutadiene polymer reacted with anorganic isocyanate and a monoethylenically unsaturated monomer carryinga single hydroxy group, to form a diethylenic diurethane having ahalogen-containing, essentially saturated polybutadiene backbone; and(b) a liquid solvent for the resin (a), this solvent being preferably amonoethylenically unsaturated liquid having a low glass transitiontemperature. The coating composition is cured to produce the finalcladding, which has a high refractive index, above 1.48.

U.S. Pat. No. 4,469,724, issued Sept. 4, 1984, describes protectingoptical fibers against stress corrosion by first coating the opticalfiber with a primary coating of an ultraviolet curable, cis, transfluoropolyolacrylate in which the modulus is reduced by eliminatingabout 25 percent of the pendant ester groups, curing the primarycoating, applying a secondary coating of a high-modulus, heat-curablefluoroepoxy or a high-modulus ultraviolet-curable cis, transfluoropolyolacrylate over the primary coating, and curing the secondarycoating. The cis, trans fluoropolyolacrylates used must contain aromaticand oxirane rings.

However, in some cases it is possible to form the primary cladding ofthe optical fiber (i.e., the cladding immediately adjacent the core)from a polymeric material. According to the aforementioned Blyler andAloisio article, polymer-clad fibers usually consist of a silica coreclad with either a poly(dimethylsiloxane) resin or a fluorinated acrylicpolymer. For example, U.S. Pat. No. 4,568,566, issued Feb. 4, 1986,describes photocurable silicone compositions, useful as optical fibercladdings, which compositions contain chemically combined siloxy unitsand units of the formula R₂ SiO, where a number of the R units areacrylate or alkyl-substituted acrylate ester groupings.

U.S. Pat. No. 4,554,339, issued Nov. 19, 1985, and U.S. Pat. No.4,597,987, issued Jul. 1, 1986, describe organopolysiloxanes having aviscosity of 100 mPa. at 25° C. and having both SiC-bonded acryloxyalkylgroups and Si-bonded hydrogen atoms in the same molecule. Theseorganopolysiloxanes are prepared by adding an allyl alcohol to adiorganopolysiloxane containing a terminal Si-bonded hydrogen atom, thenesterifying the hydroxyl groups of the resultant reaction product withacrylic acid and subsequently equilibriating the resultantdiorganopolysiloxane with an organo(poly)siloxane containing anSi-bonded hydroxyl group in each of the terminal units. The finalorganopolysiloxane is stated to be useful as, inter alia, an opticalfiber cladding, although it is not clear whether this refers to aprimary or secondary cladding.

However, silicone primary claddings have a number of seriousdisadvantages. The viscosity and curing requirements of the siliconesrestrict the production rate of the clad fiber to about 0.5 meters/sec.Silicone claddings do not adhere well to quartz, and the softness of thecladding leads to difficulties in connecting the clad fiber to othercomponents of the optical system; temperature changes can cause thequartz core to be forced into and out of the cladding at the connection.Furthermore, according to U.S. Pat. No. 4,511,209, exposing thesilicone-clad optical fibers to low temperatures in the range of -40° to-50° C. results in an increase in attenuation of 10-20 dB/km.; in manycases an increase in room temperature attenuation occurs after the fiberhas been exposed to such low temperatures.

It is also known that fluorine-containing materials can be incorporatedinto claddings containing acrylates and methacrylates. For example, U.S.Pat. No. 4,508,916, issued Apr. 2, 1985, describes curable substitutedurethane acrylates and methacrylates having an aliphatic backbone withat least one pendant fluorinated organic group attached thereto, thisbackbone being end-capped with an acrylic or methacrylic group.

U.S. Pat. No. 4,617,350, issued Oct. 14, 1986, describes a thermoplasticresin useful for optical purposes, including optical fiber claddings,and obtained by blending a polymer of an acrylic ester with a copolymerof vinylidene fluoride and hexafluoroacetone. The refractive index ofthe blend is in the range of 1.37 to 1.48.

European Patent Application Publication No. 196,212, published Oct. 1,1986, describes a curable adhesive composition suitable for splicingoptical fibers, or connecting optical fibers to other optical elements.This composition comprises a fluoroacrylate having the formula:

    (R.sub.1).sub.2 CH(CF.sub.2).sub.n CH(R.sub.2).sub.2

wherein R₁ and R₂ are acrylate, methacrylate or hydrogen, n is aninteger of 1 through 5, and at least one R₁ is acrylate or methacrylateand at least one R₂ is acrylate or methacrylate. The composition mayalso contain a polyfunctional acrylate or methacrylate monomer having 2to 7 acrylate or methacrylate groups, this polyfunctional monomerallowing adjustment of the refractive index of the adhesive compositionto precisely match the optical fiber refractive index. The compositionmay also contain an acrylate or methacrylate oligomer which does notaffect the refractive index and is used to adjust the viscosity of theuncured composition to the desired level.

U.S. Pat. No. 4,511,209, issued Apr. 16, 1985 to Skutnik, describes acurable composition for use in cladding optical fibers, this compositioncomprising (a) a highly fluorinated monofunctional acrylate with arefractive index below 1.38 and constituting more than 50 percent byweight of the composition; (b) a trifunctional or higher acrylate thatserves as a cross-linking agent; (c) a mono- or polyfunctional thiolthat functions as a synergist; and (d) a photoinitiator. The resultantcladdings have refractive indices in the range of about 1.39 to about1.43.

The use of a thiol in this composition poses obvious pollution andenvironmental problems. Thiols are notorious for their obnoxious smellsand also tend to be toxic. Although Skutnik does suggest the use ofcertain "low odor thiols", it would be advantageous to avoid the use ofthiols entirely. In addition, to produce a composition with theviscosity needed to coat an optical fiber, it is sometimes necessary toinclude a solvent in the composition (some of the fluoroacrylates aresolids at room temperature). The need for such a solvent imposesadditional costs and environmental problems, especially if a chlorinatedsolvent, such as the methylene chloride used in some of Skutnik'sExamples, is required. In addition, as noted by Blyler and Aloisio,supra, it is desirable to avoid solvents in cladding compositionsbecause solvent removal is a slow process and the cladding must solidifyquickly once it is applied to the optical fiber, in order that the fibermay be protected before it contacts any solid surface, such as thecapstan used to draw the optical fiber.

It has also been found that very hard claddings, such as those producedby the Skutnik compositions, tend to produce microbends in an opticalfiber, thus increasing the attenuation of the fiber.

There is thus a need for a photocurable composition for optical fiberswhich will produce claddings having suitably low refractive indices,which can be prepared without need for tri- or higher acrylates orthiols, and which produces coatings having a desirable degree ofsoftness. This invention provides such a photocurable composition.

SUMMARY OF THE INVENTION

This invention provides a photopolymerizable composition capable ofbeing polymerized upon exposure to ultraviolet light, the compositionforming upon photocuring a cured composition having a refractive indexnot greater than about 1.43, and comprising a substantially homogeneousmixture of:

a) an unsubstituted or fluorosubstituted diacrylate monomer;

b) a fluorinated monofunctional acrylate monomer in an amount of fromabout 2 to about 12 parts by weight per part by weight of the diacrylatemonomer;

c) a photoinitiator; and

d) a viscosity modifying agent in an amount sufficient to increase theviscosity of the composition to a value in the range of from about 1000to about 15000 cP.

This invention also provides a process for cladding an optical fiber,which process comprises contacting said fiber with a photopolymerizablecomposition comprising a substantially homogeneous mixture of:

a) an unsubstituted or fluorosubstituted diacrylate monomer;

b) a fluorinated monofunctional acrylate monomer in an amount of fromabout 2 to about 12 parts by weight per part by weight of the diacrylatemonomer;

c) a photoinitiator; and

d) a viscosity modifying agent in an amount sufficient to increase theviscosity of the composition to a value in the range of from about 1000to about 15000 cP.,

thereby coating the optical fiber with a layer of the photopolymerizablecomposition, and thereafter exposing the coated optical fiber toultraviolet light, thereby curing the photopolymerizable composition toproduce on the optical fiber a cladding having a refractive index notgreater than about 1.43.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing shows schematically an apparatus which can beused to carry out the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The diacrylate monomer used in the compositions of the present inventionmay be unsubstituted or fluorosubstituted. When unsubstituted, thisdiacrylate monomer is desirably a diacrylate of an alpha,omega alkylenediol containing from about 4 to about 12 carbon atoms, preferably thediacrylate of 1,6-hexanediol (this diacrylate will hereinafter bereferred to as HDDA). When the diacrylate monomer is fluorosubstituted,it desirably contains from about 25 to about 60 percent by weightfluorine. A preferred group of fluorosubstituted diacrylate monomers arethose of the formulae:

    CH.sub.2 ═CH--CO--OCH.sub.2 --X--CH.sub.2 --O--CO--CH═CH.sub.2

and

    X'--SO.sub.2 N(CH.sub.2 CH.sub.2 --O--CO--CH═CH.sub.2).sub.2

where X is a perfluoroalkylene grouping, or a perfluoroalkylene groupingin which one or more carbon atoms have been replaced by --O--linkages,and X' is a perfluoroalkyl group. Diols suitable for preparing thesediacrylates are available from Minnesota Mining and ManufacturingCompany, St. Paul, Minn., and below, the tradenames of these diols aregiven alongside the formulae of the diacrylates. Specific diacrylates ofthe foregoing formulae suitable for use in the present invention are:##STR1## A specific preferred fluorosubstituted diacrylate monomer is2,2,3,3,4,4,5,5-octafluorohexamethylene diacrylate, hereinafter referredto simply as octafluorohexamethylene diacrylate; when used inassociation with other compatible components, as described below, thiscompound has been found to give claddings with low refractive indicesand a desirable degree of softness.

Whether an unsubstituted or fluorosubstituted diacrylate monomer isdesirable in any specific composition of the present invention dependsupon the refractive index and hardness required in the cured cladding.Fluorodiacrylate monomers produce claddings with lower refractiveindices than claddings produced from similar compositions containingunsubstituted monomers, the refractive index decreasing as theproportion of fluorine in the fluorodiacrylate monomer increases.

The fluorinated monofunctional acrylate monomer used in the compositionsof the present invention should be one in which a minimum of three C--Fbonds exist and in which at least 25 percent of the C--H bonds in thecorresponding unsubstituted monomer have been replaced with C--F bonds.Desirably, the monomer will contain from about 30 to about 65 percent byweight of fluorine. Preferred fluorinated monofunctional acrylatemonomers are those of the formula:

    CH.sub.2 ═CH--CO--OCH.sub.2 --Y--T

where Y is a perfluoroalkylene grouping and T is fluorine or a --CF₂ Hgroup; a specific preferred monomer of this formula is1H,1H-pentadecafluorooctyl acrylate. The fluorinated monofunctionalacrylate monomer may also contain heteroatoms such as sulfur, oxygen andnitrogen; examples of such monomers are those of the formula:

    Z--SO.sub.2 --NR--CH.sub.2 --CH.sub.2 --O--CO--CA═CH.sub.2

where Z is H(CF₂)_(m) or F(CF₂)_(m), where m is an integer from 3 to 12,R is an alkyl group and A is hydrogen or methyl. Examples ofcommercially available monofunctional acrylate monomers which are usefulin the present process (the figures in parentheses are the refractiveindex of the homopolymers) are 1H,1H,5H-octafluoropentylacrylate(1.380), trifluoroethylacrylate (1.407), and heptafluorobutylacrylate(1.367), all of which are available from PCR Incorporated, P.O. Box1466, Gainesville, Fla. 32602, and ##STR2## which are available fromMinnesota Mining and Manufacturing Company, St. Paul, Minn. under thetradenames L-9911 and FX-13 respectively.

Since the diacrylate monomer acts as a crosslinking agent during curingof the composition, the ratio of diacrylate monomer to monofunctionalacrylate monomer affects the hardness of the cured cladding, thehardness increasing with the proportion of diacrylate monomer used. Thecompositions of the present invention comprise from about 2 to about 12,and preferably about 3 to about 9 parts by weight of the fluorinatedmonofunctional acrylate monomer per part by weight of the diacrylatemonomer. The ratio of diacrylate monomer to monofunctional acrylatemonomer also affects the refractive index of the cured cladding, withthis index increasing with the proportion of diacrylate monomer in thecomposition.

The photoinitiator used in the compositions of the present invention maybe any of the photoinitiators conventionally used for curing acrylatesusing ultraviolet radiation, and numerous such photoinitiators will beknown to those skilled in the art. Examples of such photoinitiators arethose of the benzoin ether type, such as benzoin methyl ether, benzoinethyl ether or benzoin isopropyl ether; substituted acetophenones suchas 2,2-diethoxyacetophenone or 2,2-dimethoxy-2-phenylacetophenone; andsubstituted α-ketols such as 2-hydroxy-2-methylpropiophenone or1-hydroxycyclohexyl phenyl ketone. One specific photoinitiator which hasbeen found to give good results in the present compositions is2,2-dimethoxy-2-phenylacetophenone (DMPA), which is availablecommercially from Aldrich Chemical Company and Ciba-Geigy. Thephotoinitiators are used in conventional amounts, typically from about0.5 to 5 percent by weight of the total composition, with the optimumamount usually being around 2 percent by weight of the totalcomposition.

A viscosity modifying agent is used in the compositions of the presentinvention in an amount sufficient to increase the viscosity of thecompositions to a value in the range of from about 1000 to about 15000cP. at 25° C. Without the viscosity modifying agent, the compositionshave viscosities so low that simply passing an optical fiber through abath of the composition will not produce an even coating of sufficientthickness of the composition to produce a useful cladding. Desirably,the amount of viscosity modifying agent used is sufficient to increasethe viscosity of the composition to about 3000 to about 10000 cP. at 25°C. The viscosity modifying agent typically comprises from about 10 toabout 25 percent by weight of the total composition.

It may sometimes be desirable, in order to produce coatings of varyingthickness, to formulate a series of differing compositions having thesame components but in which the amount of viscosity modifying agentvaries, so as to vary the viscosity of the composition. In order toprepare such a series of differing compositions, it is convenient to mixthe appropriate diacrylate monomer, fluorinated monofunctional acrylatemonomer, and photoinitiator to form a pre-mixture, which can later beadmixed with the desired amount of viscosity modifying agent.

The preferred viscosity modifying agents for use in the presentcompositions are fluorocarbon polymers having refractive indices notgreater than about 1.40, especially copolymers of vinylidene fluorideand hexafluoropropylene, and terpolymers of vinylidene fluoride,hexafluoropropylene and tetrafluoroethylene. These copolymers andterpolymers are available commercially from Minnesota Mining andManufacturing Company, St. Paul, Minn., under the trademark Fluorel, andfrom other companies. In general, in these polymers, the weight ratio ofvinylidene fluoride to hexafluoropropylene is in the range of from2.33:1 to 0.67:1, while the terpolymers generally contain from 3 to 35percent by weight of tetrafluoroethylene and from 97 to 65 percent byweight of vinylidene fluoride and hexafluoropropylene. Within theseweight ratios, the polymers are elastomeric.

These Fluorel polymers can be prepared by the copolymerization in knownmanner of a mixture of the corresponding monomers. An aqueous redoxpolymerization system can be used and polymerization can be initiated byresort to a conventional ammonium persulfate/sodium bisulfite system.Polymerization will normally be accomplished under pressure atmoderately elevated temperatures. Suitable methods for the production ofthe polymers are known and are described in greater detail in U.S. Pat.No. 2,968,649, issued Jan. 17, 1961.

A specific preferred copolymer is that sold as Fluorel FC-2175. Thismaterial is stated by the manufacturer to be of the formula:

    (--CF.sub.2 CF(CF.sub.3)).sub.n (CH.sub.2 CF.sub.2)--m

where m/n is approximately 4. The material has a refractive index of1.370 and a glass transition temperature of -22° C.

Commercial forms of fluoropolymers may contain minor components producedas by-products during the synthesis of the polymers, or suited to aparticular purpose but which may contribute to cloudiness and which areunsuitable for optical applications. These materials can, however, befiltered prior to use for removal of such components. It has been foundthat filtering a 5 percent solution of Fluorel FC-2175 in acetone undermoderately elevated pressure through diatomaceous earth, or filtering a25 percent solution of Fluorel FC-2175 in acetone through a 0.2 μpleated nylon membrane, followed by evaporation of the acetone, gives aclarified product suitable for use in the present invention.

As already noted, unlike the compositions described in Skutnik, thecompositions of the present invention do not require the incorporationof a thiol as a synergist, and there is no reason to incorporate anythiol in the present compositions. Accordingly, in view of the knownproblems associated with the use of thiols, it is strongly recommendedthat the present compositions be essentially free from thiols, i.e.,that the compositions not contain an amount of a thiol whichsignificantly affects the hardness of the cured composition.

It is important to ensure that the components of the presentcompositions are compatible with one another, such that not only do allthe components form a single, substantially homogeneous phase prior tocuring, but also that a single, substantially homogeneous solid phase isformed after curing. This single solid phase is believed to be a solidsolution of the viscosity modifying agent in a copolymer of thediacrylate monomer with the fluorinated monofunctional acrylate monomer.The formation of a single, substantially homogeneous solid phase aftercuring is necessary to produce a clear cladding, since a cloudy claddingwill greatly increase attenuation in an optical fiber.

Cladding of optical fibers by the process of the present invention maybe effected using any of the conventional techniques known to thoseskilled in the art; the presently preferred technique is described inthe Examples below.

The compositions of the present invention may be used for cladding anyoptical fibers. They may, for example, be used for cladding the fibersof optical fiber lasers, such as those described in U.S. Pat. No.4,815,079, issued Mar. 21, 1989 to Snitzer et al. The fiber optic lasersdescribed in this patent comprise a single-mode core disposed within arelatively large, multimode cladding such that the core is displacedfrom the center of the cross-section of the cladding. The cladding issurrounded by a further layer (second cladding) to prevent radiationfrom propagating out of the cladding. The compositions of the presentinvention may be used to form the second cladding of such a opticalfiber laser, and permit the refractive index of this cladding to be suchas to produce a numerical aperture (given by:

    N.A.=(N.sub.1.sup.2 -N.sub.2.sup.2).sup.1/2

where N₁ is the refractive index of the first cladding and N₂ that ofthe second cladding) for the optical fiber of at least about 0.20.Indeed, preferred embodiments of the present invention can achievenumerical apertures of about 0.4 in such an optical fiber laser. Thepresent compositions can, it has been found experimentally, give verysatisfactory claddings even on optical fiber lasers of the form shown inFIG. 2 of the patent, in which the optical fiber laser is of ellipticalrather than circular cross-section.

The present invention thus provides a photocurable composition forcladding optical fibers or other articles, which will produce claddingshaving refractive indices below 1.43; the refractive indices of some ofthe cured compositions are below 1.38. The cured claddings can be madesufficiently soft to avoid microbending of the clad fiber, but harderthan conventional silicone claddings. The process of the presentinvention avoids the use of tri- or higher acrylates and thiols.Furthermore, the compositions of the present invention have viscositieswhich permit coatings of proper thickness to be produced on opticalfibers without a need to introduce solvent into the composition.

The following Examples are now given, though by way of illustrationonly, to show details of preferred reagents, conditions and techniquesused in the present invention.

EXAMPLES Example 1: Preparation of octafluorohexamethylene diacrylate

This Example illustrates the preparation of a preferred diacrylatemonomer for use in the process of the present invention.

50 g. (0.191 mole) of octafluorohexamethylene diol, 400 ml. of methylenechloride and 65 ml. (47.2 g., 0.466 mole) of triethylamine were placedin a 1000 ml. three-necked, round-bottomed flask equipped with amagnetic stirrer, a thermometer, a 250 ml. addition funnel and acondenser. The resultant solution was cooled with stirring in anice-water bath to 5° C., and then a solution of 35 ml. acryloyl chloride(39.0 g., 0.431 mole) in 200 ml. of methylene chloride was addeddropwise through the addition funnel at a rate of approximately one dropper second over a period of 3 hours, while the temperature of thesolution was maintained at 5° C. Although a white solid began toprecipitate after about half the acryloyl chloride had been added, thepresence of this solid did not interfere with stirring.

After addition of the acryloyl chloride was complete, the resultantsolution was stored in a refrigerator over a weekend, then washedsuccessively with 200 ml. of water, 200 ml. of half-saturated sodiumbicarbonate solution, 200 ml. of 5 percent hydrochloric acid and 200 ml.of water. The organic phase obtained after washing was filtered throughcotton and the methylene chloride evaporated in a rotary evaporator toyield an amber oil. Analysis of this oil by gas chromatography on apolydimethylsiloxane packed column at a temperature of 125-150° C. withpassage of 30 ml/min. of helium showed 91 percent diester and less than1 percent monoester.

After addition of 10-20 mg. of di-tert-butylhydroquinone as apolymerization inhibitor, the product was distilled in a 250 ml.three-necked, round-bottomed flask equipped with a magnetic stirrer, aheating mantle, a thermometer, a 6 inch (152 mm.) Vigreux column and avariable reflux ratio still head connected to a vacuum pump through amanometer and a liquid nitrogen trap. Several milliliters of low boilingfractions (b.pt. 50°-90° C. at 0.5 mm.) were collected first, and then,at a reaction pot temperature of about 115° C., the diacrylate productdistilled at 92-96° C. at 0.5 mm.; the pressure was not quite constantbecause the column tended to flood. At the end of the distillation, thedark brown residue in the flask polymerized to a fluffy, acetoneswellable material.

The product was 60.1 g. (85 percent of theoretical) of a pale yellowliquid having n_(D) ²¹ =1.3907 and a specific gravity of 1.43 at 21° C.The product analyzed as 95 percent diacrylate by gas chromatography, andits identity was confirmed by infrared, and proton and fluorine NMRspectroscopy.

The reaction proceeds according to the equation: ##STR3##

Example 2: Purification of viscosity modifying agent

This Example illustrates the purification of a commercially availablepolymer to produce a preferred viscosity modifying agent for use in thepresent invention.

25 g. of Fluorel FC-2175 (obtained from Minnesota Mining andManufacturing Company, St. Paul, Minn.) were dissolved in 500 ml. ofreagent grade acetone in a 1000 ml. Erlenmeyer flask; the Fluorel willdissolve overnight without stirring. A 1000 ml. flash chromatographycolumn 50 mm. in diameter was slurry packed under house line nitrogenpressure (less than 15 psig.) with 25 g. of diatomaceous earth, usingacetone as the liquid component of the slurry, to produce a layer ofdiatomaceous earth approximately 40 mm. thick.

The acetone solution of Fluorel was then filtered, under line nitrogenpressure, through the packed column; the column removed residual saltsand other impurities. The column was washed several times withadditional quantities of acetone, and all the eluates from the columnwere combined.

The combined eluates were then transferred to a 1000 ml. heavy walled,pear-shaped flask and evaporated on a rotary evaporator using a bathkept below 40° C. until about 100 ml. of the acetone solution remained.The resultant viscous solution was transferred to a pre-weighed 250 ml.round-bottomed flask, the residue remaining in the pear-shaped flaskquantitatively rinsed into the round-bottomed flask with ethyl acetate,and the remaining solvents were removed on the rotary evaporator, carebeing taken to avoid bumping during this evaporation.

The round-bottomed flask was then placed in an unheated vacuum oven anddried to constant weight under full oil pump vacuum. The resultantpurified Fluorel weighed approximately 25 g. and was pale yellow incolor.

Example 3: Purification of viscosity modifying agent

This Example illustrates an alternative purification of a commerciallyavailable polymer to produce a preferred viscosity modifying agent foruse in the present invention.

A 15 percent solution of Fluorel FC-2175 in reagent grade acetone wasprepared and pressure-filtered through a 0.2 μ nylon cartridge filter.The clear filtrate was concentrated on a rotary evaporator under reducedpressure and was then dried under vacuum to yield the desired purifiedmaterial.

Example 4: Preparation and use of cladding composition

This Example illustrates the preparation of a preferred claddingcomposition of the present invention containing a fluorosubstituteddiacrylate monomer, and the application of this cladding composition toan optical fiber.

115.4 g. of pentadecafluorooctyl acrylate sold by Minnesota Mining andManufacturing Company, St. Paul, Minn. under the tradename FC-5165),36.0 g. of octafluorohexamethylene diacrylate (prepared in Example 1above) and 3.6 g. of DMPA were mixed in a beaker and warmed over a steambath in a fume hood until a homogeneous solution was obtained. Thissolution was poured into a 250 ml. round-bottomed flask containing the25 g. of purified Fluorel produced in Example 2 above, and a magneticstirring bar was placed in the flask. The flask was then placed in anoil bath located in a fume hood with its lights off (to preventpremature curing of the mixture), this oil bath being provided with athermometer and disposed on a combination magnetic stirrer/hot plate.The bath was heated to approximately 120° C. and the mixturemagnetically stirred for several hours until a homogeneous, lightyellow, viscous solution was obtained. While still hot, the solution wascarefully transferred into a 100 ml. dark amber bottle with a Polysealcap for storage. The resultant clear, light yellow solution was of lowvolatility and had a Brookfield viscosity of approximately 4500 cP. at25° C.

To test the refractive index of the cured form of this solution, two 25mm. by 75 mm. glass microscope slides were spaced about 0.5 mm. apart.The gap between the slides was filled with the solution, which was curedin air for less than 1 minute approximately 5 cm. beneath a water-cooled450 Watt ultraviolet mercury vapor lamp. The solution cured to produce aclear, soft film with n_(D) ²⁰ =1.378.

The solution was coated onto optical fibers using the apparatus shownschematically in the accompanying drawing. This apparatus, generallydesignated 10, comprised an oven 12 containing a glass preform. Beneaththe oven 12 were disposed two coating cups 14 and 16, each containingthe cladding solution prepared above. An ultraviolet lamp 18 (a FusionResearch electrodeless ultraviolet mercury vapor lamp) was disposedbelow the coating cup 16, and a capstan 20 was disposed below the lamp18. The apparatus further comprised a wind-up roll 22.

An optical fiber 24 was drawn from the oven 12 at a rate ofapproximately 0.5 m/sec. by the capstan 20, and passed through thecoating cups 14 and 16. These cups were provided withdownwardly-tapering conical bases, with the apex of each cone having avertical bore passing therethrough, the diameter of this bore beingequal to the desired diameter of the optical fiber coated with thecladding solution, so that the bore wipes excess cladding solution fromthe fiber. The fiber, with the uncured cladding solution thereon, wasthen traversed past the lamp 18, where the solution was cured to producean adherent clear, soft cladding on the optical fiber.

When the above procedure was used to coat the core and first cladding ofan optical fiber laser as shown in FIG. 2 of the aforementioned U.S.Pat. No. 4,815,079, this first cladding having an ellipticalcross-section of approximately 110 by 40 μ, the cured composition of thepresent invention formed a coherent layer approximately 15 μ thicksurrounding the first cladding. The numerical aperture of the resultantoptical fiber was approximately 0.4.

Example 5: Preparation and use of cladding composition containingunsubstituted diacrylate monomer

This Example illustrates how a cladding composition of the presentinvention containing an unsubstituted diacrylate monomer could beprepared.

A cladding solution is prepared in the same way as in Example 3 butusing 10 parts by weight of 1,6-hexanediol diacrylate (obtained fromAldrich Chemical Co.) as the diacrylate monomer, 74 parts by weight ofpentadecafluorooctyl acrylate as the fluorinated monofunctional acrylatemonomer, 2 parts by weight of DMPA as the photoinitiator and 14 parts byweight of purified Fluorel FC-2175 as the viscosity modifying agent. Theresultant cladding solution may be applied to optical fibers using thesame technique as in Example 3.

Attention is directed to the copending application of Richard A. Minns,Ser. No. 07/521,642, of even date entitled "Photopolymerizablecomposition for cladding optical fibers" [Applicant's reference C-7602]which discloses cladding compositions related to those used in theprocess of the present invention, and processes for the use of suchcompositions.

We claim:
 1. A photopolymerizable composition capable of beingpolymerized upon exposure to ultraviolet light, the composition formingupon photocuring a cured composition having a refractive index notgreater than about 1.43, and comprising a substantially homogeneousmixture of:a) an unsubstituted or fluorosubstituted diacrylate monomer;b) a fluorinated monofunctional acrylate monomer in an amount of fromabout 2 to about 12 parts by weight per part by weight of the diacrylatemonomer; c) a photoinitiator; and d) a viscosity modifying agent in anamount sufficient to increase the viscosity of the composition to avalue in the range of from about 1000 to about 15000 cP.
 2. Acomposition according to claim 1 wherein the fluorosubstituteddiacrylate monomer contains at least about 25 percent by weightfluorine.
 3. A composition according to claim 1 wherein thefluorosubstituted diacrylate monomer comprises at least one compound ofone of the formulae:

    CH.sub.2 ═CH--CO--OCH.sub.2 --X--CH.sub.2 --O--CO--CH═CH.sub.2

and

    X'--SO.sub.2 N(CH.sub.2 CH.sub.2 --O--CO--CH═CH.sub.2).sub.2

where X is a perfluoroalkylene grouping, or a perfluoroalkylene groupingin which one or more carbon atoms have been replaced by --O--linkages,and X' is a perfluoroalkyl group.
 4. A composition according to claim 3wherein the fluorosubstituted diacrylate monomer comprisesoctafluorohexamethylene diacrylate.
 5. A composition according to claim1 wherein the diacrylate monomer is an unsubstituted diacrylate monomer.6. A composition according to claim 5 wherein the unsubstituteddiacrylate monomer comprises at least one diacrylate of an alpha,omegaalkylene diol containing from about 4 to about 12 carbon atoms.
 7. Acomposition according to claim 6 wherein the unsubstituted diacrylatemonomer comprises 1,6-hexanediol diacrylate.
 8. A composition accordingto claim 1 wherein fluorinated monofunctional acrylate monomer comprisesat least one compound of the formula:

    CH.sub.2 ═CH--CO--OCH.sub.2 --Y--T

where Y is a perfluoroalkylene grouping and T is fluorine or a --CF₂ Hgroup.
 9. A composition according to claim 8 wherein fluorinatedmonofunctional acrylate monomer comprises pentadecafluorooctyl acrylate.10. A composition according to claim 1 comprising from about 3 to about9 parts by weight of the fluorinated monofunctional acrylate monomer perpart by weight of the diacrylate monomer.
 11. A composition according toclaim 1 wherein the viscosity modifying agent comprises a fluorocarbonpolymer having a refractive index not greater than about 1.40.
 12. Acomposition according to claim 11 wherein the viscosity modifying agentcomprises a copolymer of vinylidene fluoride an hexafluoropropylene, ora terpolymer of vinylidene fluoride, hexafluoropropylene andtetrafluoroethylene.
 13. A composition according to claim 11 wherein theviscosity modifying agent comprises from about 10 to about 25 percent byweight of the total composition.
 14. A composition according to claim 1which is essentially free from thiols.
 15. A process for cladding anoptical fiber, which process comprises contacting the fiber with aphotopolymerizable composition comprising a substantially homogeneousmixture of:a) an unsubstituted or fluorosubstituted diacrylate monomer;b) a fluorinated monofunctional acrylate monomer in an amount of fromabout 2 to about 12 parts by weight per part by weight of the diacrylatemonomer; c) a photoinitiator; and d) a viscosity modifying agent in anamount sufficient to increase the viscosity of the composition to avalue in the range of from about 1000 to about 15000 cP.,thereby coatingthe optical fiber with a layer of the photopolymerizable composition,and thereafter exposing the coated optical fiber to ultraviolet light,thereby curing the photopolymerizable composition to produce on theoptical fiber a cladding having a refractive index not greater thanabout 1.43.
 16. A process according to claim 15 wherein thefluorosubstituted diacrylate monomer contains at least about 25 percentby weight fluorine.
 17. A process according to claim 15 wherein thefluorosubstituted diacrylate monomer comprises at least one compound ofone of the formulae:

    CH.sub.2 ═CH--CO--OCH.sub.2 --X--CH.sub.2 --O--CO--CH═CH.sub.2

and

    X'--SO.sub.2 N(CH.sub.2 CH.sub.2 --O--CO--CH═CH.sub.2).sub.2

where X is a perfluoroalkylene grouping, or a perfluoroalkylene groupingin which one or more carbon atoms have been replaced by --O--linkages,and X' is a perfluoroalkyl group.
 18. A process according to claim 17wherein the fluorosubstituted diacrylate monomer comprisesoctafluorohexamethylene diacrylate.
 19. A process according to claim 15wherein the diacrylate monomer is an unsubstituted diacrylate monomer.20. A process according to claim 19 wherein the unsubstituted diacrylatemonomer comprises at least one diacrylate of an alpha,omega alkylenediol containing from about 4 to about 12 carbon atoms.
 21. A processaccording to claim 20 wherein the unsubstituted diacrylate monomercomprises 1,6-hexanediol diacrylate.
 22. A process according to claim 15wherein the fluorinated monofunctional acrylate monomer comprises atleast one compound of the formula:

    CH.sub.2 ═CH--CO--OCH.sub.2 --Y--T

where Y is a perfluoroalkylene grouping and T is fluorine or a --CF₂ Hgroup.
 23. A process according to claim 22 wherein the fluorinatedmonofunctional acrylate monomer comprises pentadecafluorooctyl acrylate.24. A process according to claim 15 wherein the photopolymerizablecomposition comprises from about 3 to about 9 parts by weight of thefluorinated monofunctional acrylate monomer per part by weight of thediacrylate monomer.
 25. A process according to claim 15 wherein theviscosity modifying agent comprises a fluorocarbon polymer having arefractive index not greater than about 1.40.
 26. A process according toclaim 25 wherein the viscosity modifying agent comprises a copolymer ofvinylidene fluoride and hexafluoropropylene, or a terpolymer ofvinylidene fluoride, hexafluoropropylene and tetrafluoroethylene.
 27. Aprocess according to claim 25 wherein the viscosity modifying agentcomprises from about 10 to about 25 percent by weight of the totalphotopolymerizable composition.
 28. A process according to claim 15wherein the photopolymerizable composition is essentially free fromthiols.
 29. A process according to claim 15 wherein the claddingproduced has a refractive index of not more than about 1.40.
 30. A cladoptical fiber produced by a process according to claim
 15. 31. Anoptical fiber having a cladding comprising a solid solution of acopolymer of vinylidene fluoride and hexafluoropropylene, or aterpolymer of vinylidene fluoride, hexafluoropropylene andtetrafluoroethylene in a copolymer of an unsubstituted orfluorosubstituted diacrylate monomer with a fluorinated monofunctionalacrylate monomer, the copolymer comprising from about 2 to about 12parts by weight of the fluorinated monofunctional acrylate monomer perpart by weight of the diacrylate monomer, the cladding having arefractive index of not more than about 1.43.
 32. An optical fiberaccording to claim 31 wherein the cladding has a refractive index of notmore than about 1.40.
 33. A photopolymerizable composition capable ofbeing polymerized upon exposure to ultraviolet light, the compositioncomprising a substantially homogeneous mixture of:a) an unsubstituted orfluorosubstituted diacrylate monomer; b) a fluorinated monofunctionalacrylate monomer in an amount of from about 2 to about 12 parts byweight per part by weight of the diacrylate monomer; c) aphotoinitiator.
 34. A photopolymerizable composition according to claim33 wherein the diacrylate monomer is an unsubstituted diacrylatemonomer.